In recent years, attention has been paid to using hydrogen as fuel because there is no generation of global warming gas such as carbon dioxide from the aspect of the global environmental protection and the energy efficiency is high. Particularly, inasmuch as fuel cells can directly convert hydrogen into electric power and enable high energy conversion efficiency in cogeneration systems utilizing generated heat, attention has been paid thereto. Heretofore, the fuel cells have been employed under a special condition such as in the space development or the ocean development. Recently, however, the development has been advanced toward using them as automobile or household distributed power supplies. Further, fuel cells for portable devices have also been developed.
The fuel cell is a power generator wherein hydrogen rich gas obtained by reforming hydrocarbon fuel such as natural gas, gasoline, butane gas, or methanol, and oxygen in the air are reacted electrochemically, thereby to directly produce electricity. In general, the fuel cell comprises a reformer for producing hydrogen rich gas by steam-reforming hydrocarbon fuel, a fuel cell body for producing electricity, a converter for converting the produced dc electricity into alternating current, and so forth.
Depending on an electrolyte used in the fuel cell body, a reaction manner, and so forth, there are five kinds in those fuel cells, i.e. a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), an alkaline fuel cell (AFC), and a solid polymer fuel cell (PEFC). Among them, the solid polymer fuel cell (PEFC) has a favorable condition in that an electrolyte is solid, as compared with other fuel cells such as the phosphoric acid fuel cell (PAFC) and the alkaline fuel cell (AFC).
However, since the solid polymer fuel cell (PEFC) uses platinum as a catalyst and an operating temperature thereof is low, there is a drawback that the electrode catalyst is poisoned with a small quantity of CO, and degradation in performance is remarkable particularly in a high current density region. Therefore, it is necessary to produce high purity hydrogen by reducing a concentration of CO contained in reformed gas (hydrogen rich gas) produced in a reformer, to about 10 ppm.
As one of methods for removing CO from the reformed gas, there has been used a membrane separation method employing a Pd alloy film as a filter. Unless there are pinholes, cracks, or the like in the film, the Pd alloy film can theoretically transmit only hydrogen and, by setting the reformed gas side under high-temperature and high-pressure conditions (e.g. 300° C., 3 to 100 kg/cm2), it transmits hydrogen to the low hydrogen partial pressure side.
In the foregoing membrane separation method, inasmuch as the transmission speed of hydrogen is inversely proportional to a film thickness, reduction in film thickness is required. However, in terms of the mechanical strength, reduction in film thickness up to about 30 μm is a limit for a Pd alloy film alone, and therefore, when a Pd alloy film having a thickness of about ten-odd micrometers is used, a support member having a porous structure is disposed on the low hydrogen partial pressure side of the Pd alloy film. However, since the Pd alloy film and the support member are mounted in a reformer as separate members, there has been a problem that the operability for achieving excellent sealing is bad, and durability of the Pd alloy film is not sufficient due to occurrence of friction between the Pd alloy film and the support member.
For solving the foregoing problem, there has been developed a filter in which a Pd alloy film and a support member of a porous structure are unified together using an adhesive. However, there has been a problem that it is necessary to remove the adhesive from the Pd alloy film located at hole portions of the support member, and therefore, the production processes are complicated. Further, since it is used under high-temperature and high-pressure conditions in the reformer, degradation of the adhesive is unavoidable, resulting in insufficient durability of the filter.
Moreover, there is a limit to the magnitude of opening diameters of hole portions in the support member for ensuring a required strength of the support member, and therefore, there is also a limit about increasing an area of the Pd alloy film that is effective for transmission of hydrogen, so that improvement in hydrogen transmission efficiency has been impeded.